The Tamron at 300mm & f/5.6 appears to be "holding it's own" quite well in the center, and may even be more crisp mid-frame & in the corners, against the mighty Canon 70-200 II (at a similar f/5.6) with a 1.4x TC.

So if you already have a Canon 70-200mm f/2.8 L IS II (& you wanted to go longer) for about the same cost of both Canon Extenders, you can get the extra 400-600mm range and better sharpness by going with the Tamron?

If I'm understanding all this correctly, for someone like me who typically photographs "static" landscapes, this is a no-brainer right? At least until the Canon 100-400mm II becomes available...

Looking at the TDP samples, the Tammy looks like it could be an interesting addition to my lens collection, where my long lenses include the 135F2L, 70-200F4L IS, and both TC IIIs. As expected the Tammy cannot match the 135F2L in sharpness, even versus the 1.4x at larger apertures. But the Tammy does appear to outperform the 135F2+2.0x. A comparison with the 70-200F4L IS yields similar results by itself or with the 1.4x, while the Tammy outperforms the 70-200 with the 2.0x and has an aperture advantage.

Doubt I will use the TCs as much once the Tammy arrives. Wonder how the Tammy performs with a TC? Anybody tried this yet? Might try to shoot the moon .

Yes, you are reading correctly. The 100-400 L is slightly better than the 70-200 with extenders at 280 and 400mm, and the Tamron 150-600 and the 100-400 are within a whisker of each other. Canon TCs are overpriced.

The Canon TCs won't fit on to the Tammy so I couldn't test it. But, I would guess that the Kenko etc ones will give terrible IQ and AF.

So if you already have a Canon 70-200mm f/2.8 L IS II (& you wanted to go longer) for about the same cost of both Canon Extenders, you can get the extra 400-600mm range and better sharpness by going with the Tamron?

If I'm understanding all this correctly, for someone like me who typically photographs "static" landscapes, this is a no-brainer right? At least until the Canon 100-400mm II becomes available...

Indeed... I think if this lens was released a couple of years back when i first wanted length, i would have gone for it over adding extenders to my 70-200... that resolution is inspiring. Lets see how Canon will counter it... If the putative 100-400 II does not add a significant IQ, the tamron is a clear winner, and lets face it, the next 100-400 will cost a damn sight more than the tammy!

The Tammy is really good at 500mm, wide open at f/6.3. In particular, it is sharp at the corners, unlike at 600mm. So, I think for birds in flight, it may be best used at 500mm so if the fast ones are close to the edge they will still be sharp and you get a wider angle for view for capture.

The Tammy at 500mm f/6.3 is as sharp as the Canon 100-400L at 400mm. So you get a 25% boost in reach and 56% more pixels on target with no loss of image quality.

Looking at the TDP samples, the Tammy looks like it could be an interesting addition to my lens collection, where my long lenses include the 135F2L, 70-200F4L IS, and both TC IIIs. As expected the Tammy cannot match the 135F2L in sharpness, even versus the 1.4x at larger apertures. But the Tammy does appear to outperform the 135F2+2.0x. A comparison with the 70-200F4L IS yields similar results by itself or with the 1.4x, while the Tammy outperforms the 70-200 with the 2.0x and has an aperture advantage.

Doubt I will use the TCs as much once the Tammy arrives. Wonder how the Tammy performs with a TC? Anybody tried this yet? Might try to shoot the moon .